1. Field of the Invention
This invention relates to a magnetic field measuring apparatus, and in particular to a magnetic field measuring apparatus that utilizes the nuclear magnetic resonance effect to measure a magnetic field with high accuracy.
2. Description of the Related Art
There are many methods of measuring a magnetic field. One method which is widely used now is the method using the Hall effect. As is well known, the Hall effect is a phenomenon that when a magnetic field is added perpendicularly to the surface of a substance of very high electron mobility (for example, indium arsenide (InAs)), and a constant current is supplied in a direction perpendicular to the magnetic field, a voltage is generated which is proportional to the magnitude of the magnetic field and the magnitude of the current in a direction perpendicular to both the magnetic field and the current.
The magnitude of a magnetic field can easily be measured using the Hall effect by supplying a current and then measuring the resultant voltage (which is proportional to the magnetic field). However, the accuracy of measurement is about 0.1% at best, and if higher accuracy is required, this method cannot be used.
To measure with higher accuracy, measuring devices utilizing the nuclear magnetic resonance effect (hereinbelow abbreviated as NMR) are therefore used. A detailed explanation of NMR will be omitted because it is given in documents such as "Introduction to Solid State Physics" by Charles Kittel (John Wiley & Sons, Inc., New York). However, NMR can be summarized in relevant part as follows.
A static magnetic field is added to certain kinds of substance (for example, water). Also applied is a high-frequency magnetic field of a particular frequency which is chosen based on the characteristics of the substance, and which frequency is also in proper proportion to the static magnetic field. When the frequency is properly chosen, the magnetic properties of the substance change due to a resonance effect at the atomic level. This change in magnetic properties can be detected as a change in the inductance of the coil generating the high-frequency magnetic field.
An example of an NMR magnetic field measuring apparatus utilizing this phenomenon is shown in FIG. 10. As shown in this figure, this measuring apparatus comprises a main part 10 and a probe part 20. Main part 10 comprises a high-frequency oscillator circuit 11, a detecting amplifier circuit 12, a low-frequency oscillator circuit 13, and a cathode-ray tube 14. Probe part 20 comprises a magnetic resonance substance 21 (such as water sealed into an insulating container), a high frequency coil 22 wound around the outside thereof, and bias magnetic field coils 23, 24 for generating a bias magnetic field B.sub.I that is superimposed on the external magnetic field B.sub.e that is to be measured.
The following general relationship exists between the total magnetic field B that is present in the magnetic resonance substance 21, and the frequency f.sub.R of the high-frequency magnetic field that creates the nuclear magnetic resonance: EQU B=A.times.f.sub.R ( 1)
where A is a known constant depending on the substance which can be found in reference literature, and EQU B=B.sub.e +B.sub.I ( 2)
The inductance of the high-frequency coil is different at this resonant frequency than at other frequencies. A resonance circuit comprising high-frequency oscillator circuit 11 and detecting amplifier circuit 12 detects resonance because of this variation of inductance. The voltage across the coil 22 is detected and amplified by detecting amplifier circuit 12 and is applied as e.sub.v to the vertical direction deflecting electrode of cathode ray tube 14. A ramp waveform current I output from low-frequency oscillator circuit 13 is fed to bias magnetic field coils 23 and 24, and a voltage e.sub.h proportional to I is generated by using a resistor. This voltage e.sub.h is applied to the horizonal deflecting electrode of cathode ray tube 14.
These waveforms of B.sub.I, e.sub.h and e.sub.v are shown in FIG. 11. It can be seen that resonance occurs when B.sub.I =B.sub.IR. This gives rise to resonance waveforms of e.sub.v. Because e.sub.h is proportional to B.sub.I, the resonance points within the e.sub.v waveform are synchronized with the ramp waveform of e.sub.h. The resonance waveforms are therefore displayed at fixed positions on the screen of cathode ray tube 14. The value B.sub.IR at which bias magnetic field B.sub.I causes resonance can be determined from the horizontal position (phases) of these resonance waveforms.
Therefore the external magnetic field B.sub.e that is to be measured can be calculated by substituting B.sub.I =B.sub.IR into equation (2) to derive the relation: EQU B.sub.e =A.times.f.sub.R -B.sub.IR ( 3)
In the measuring apparatus constructed as above, normally the range in amplitude of the output of the low-frequency oscillator 13 (i.e. the amplitude of variation of bias magnetic field B.sub.I) is constant, so the range of magnitudes of the external magnetic field B.sub.e that can be measured is the same as the range over which it is possible to vary this bias magnetic field B.sub.I. If it is required to measure B.sub.e over a wide range, one must perform a modification of circuit constants of the resonance circuit comprising elements 11 and 12 to change the approximation to resonant frequency f.sub.R produced by high-frequency oscillator circuit 11. Unless the value of external magnetic field B.sub.e can be predicted beforehand to some extent, this modification has to be conducted by trial and error to find a resonant frequency f.sub.R for which resonance waveforms are displayed on the cathode ray tube 14.
Furthermore, in order to measure using this measuring apparatus with high accuracy, the accuracy of the oscillation frequency of high-frequency oscillator circuit 11 must be increased, the range of output amplitude of low-frequency oscillator circuit 13 must be decreased, and the gain of the signal circuit which applies signal e.sub.h to the horizontal deflecting electrode of the cathode ray tube 14 must be increased. Disadvantageously, if the output amplitude of low-frequency oscillator circuit 13 is reduced, the range of the magnetic field which can be measured becomes small, increasing the complexity of the oscillation frequency modification. Thus, although the known measuring apparatus shown in FIG. 10 can measure magnetic fields with considerable accuracy, it has the disadvantage that time-consuming adjustment operations are required in order to obtain the measured magnetic field value.